1. Field of the Invention
The present invention relates generally to an apparatus and method for measuring the level of a fluid. More particularly, the present invention utilizes standing wave reflectivity of a coaxial transmission line to determine the level of a fluid in which the coaxial transmission line is immersed.
2. Description of the Related Art
There are many ways to measure the level of a fluid, and the techniques are numerous since fluids have many physical properties that can be exploited. These properties include the temperature, weight, capacitance, density, resistance, etc., of the fluid.
Examples of various techniques and devices include thermal fluid level sensors. Fluids have a heat capacity and thermal conductivity that is different from air. Small heating elements will have different temperatures whether they are in, or out, of the fluid. Another method uses capacitance to measure the level of the fluid. Since the dielectric constant of a fluid is different than that of air, capacitor plates having an intervening fluid will have a larger capacitance. Resistive and eddy current techniques have also been utilized. Since some fluids are electrically conducting, electrodes will carry current when immersed in a fluid. Eddy current losses for a coil will be higher when the coil is immersed in a conductive liquid.
Magnetostrictive techniques are also popular. This technique uses a combined magnetic and ultrasonic effect. It is possible to magnetically generate twist pulses in a wire (the Wiedemann Effect), which propagate at the speed of sound in the material and are reflected at the point at which the wire enters the liquid. The time of flight is used to measure the fluid level. Fiber optic technology is also utilized. The index of refraction difference between a glass fiber and the fluid can be used to optically detect the fluid level. Ultrasonic ranging is yet another method readily available. Since the time of flight of an ultrasonic pulse differs while propagating either in the fluid or in air, the difference can be used to detect fluid level. Ultrasonic damping has also been exploited. Acoustically resonant elements (e.g., ultrasonic transducers) will be damped by a fluid, that is, it will take more energy to excite them at a given amplitude. The time of flight of a radar pulse is also used to detect the surface of the fluid. This is usually not useful for small containers, although micropower impulse radar (MIR) will work at short range. Finally, a pressure sensor at the bottom of a container will gauge the weight of the fluid above it.
Though each of these techniques and sensors has a specific use, no one device or method provides an accurate fluid level measurement suitable for various applications.
It is therefore an aspect of the present invention to provide an apparatus and method for measuring the level of a fluid. The preferred embodiment of the present invention incorporates a novel approach using frequency domain analysis (FDA) of standing wave resonance (SWR) in a coaxial transmission line. It is based on the principle that the speed of light is slower in a fluid than in air. The sensor is applicable to level sensing for a wide variety of fluids, including oil and water. The sensor also functions in very harsh fluids (e.g., acids) with a proper choice of materials.